Amazed to hear this! Yep that’s actually possible, you would need only one relay and a handful of diodes to make a simplest one relay automatic battery charger circuit.
How it Works
The idea struck me while trying to design the easiest possible battery charger circuit for one my clients.
The concept is simple; just raise the operating or triggering voltage of the relay up to the optimal battery charging threshold voltage by dropping the required amount of supply voltage to the relay coil, with the help of series diodes.The idea may be understood from the following points:
Take an ordinary relay, measure its triggering voltage by carefully applying a variable voltage across its coil.Now suppose the triggering voltage of the particular relay was about 9 volts, and also assume you want to raise its voltage to 14 volts, which may be your 12 volt battery’s charging threshold voltage.
We know that a 1N4007 diode drops about 0.6 volts across it, so if we add sufficient number of diodes in series with the relay coil would hopefully pull its tripping or triggering voltage to about 14 volts.
That means, 14 – 9 = 5, we’ll require 5/0.5 = 10 diodes in series to achieve this rise in the triggering voltage of the relay.That’s pretty simple and interesting isn’t it?The rest may be done with the help of the shown diagram…..your simplest single relay automatic battery charger is ready.
Ajward says
Dear sir,
Can we use resistor or a pot instead of diodes.
Swagatam says
No, pot or resistive divider will not work instead of diodes.
M.I.Ajward says
Dear sir,
I would like to know whether it is Ok if I charge the 12v 36 ah car battery continuously with a 14v 1amp charger without auto cutoff circuit. I appreciate your reply.
Swagatam says
Hello MI,
yes it is perfectly OK. but with 1 amp your 36 Ah battery cannot charge properly, or might taken a very long time to get fully charged.
shrikant ranade says
I want some simple 5 volt relay projects with led
i also purchase pcb boards of this project
Swagatam says
You can use all the 12V relay projects from this website and convert them to 5V relay projects by replacing the 12V relay with a 5 V relay and replacing the 12V supply with 5 V supply
kavin says
How to design a relay to fix between laptop charger plug and brick and to control the relay using mobile phone
marwerno says
Hi,
exactly what I was looking for (I was always thinking of a way with Z-Diodes).
But I believe there is a small error?
You write:
“14 – 9 = 5, we’ll require 5/0.5 = 10 diodes”
0.5 represents the Voltage drop of the diode, but in the N4001 example it would be 0.6 right?
That would change the calculation to 8 Diodes?
Thanks,
Markus
Swagatam says
Thank you for your feedback and suggestions!
Grtech says
Please share a automatic charging cut off circuit for 12v 35Ah battery.
can I use 12v 2A tranfo.?
Swagatam says
you can these circuits:
https://www.homemade-circuits.com/high-current-10-to-20-amp-automatic/
https://www.homemade-circuits.com/opamp-low-high-battery-charger/
untung suharto says
Hi, friend this is can workout well charge SLA battery?
Swagatam says
yes it will work….
Farhan Ali says
yes relay starts to switch on and off. i have made this circuit.this is due to disconnection. it order to overcome this connect 14v dc to battery positive terminal not through the NC of relay.
Unknown says
Sir, thank you for sharing your idea on this simple circuit.
What components will be required if I need this same simple circuit to charge 48 volts battery made from 4 x 12. The battery AH is not important.
anticipating your quick response
Thank you once more
Swagatam says
sorry this circuit is not recommended for a 48V battery because too many diodes will be required for the 48v operation …….moreover the design is not too accurate.
Swagatam says
Actually, I was wrong in acknowledging the zener idea, it won't work.
It has to be done exactly as suggested in the diagram in the above article. 1N4007 or 1N4148 in series with the relay coil is the only correct way of doing it.
Ahmed Elmasry says
hi i take your idea and make this circuit …
im75.gulfup.com/3SLo5.png
Swagatam says
Actually, I was wrong in acknowledging the zener idea, it won't work
It has to be done exactly as suggested in the diagram in the above article. 1N4007 or 1N4148 in series with the relay coil is the only correct way of doing it.
Ahmed Elmasry says
i take your idea and made this circuit :im75.gulfup.com/3SLo5.png
rohit choudhary says
Please share a automatic cut off circuit so that , I can charge a mobile battery , with mobile charger.
Swagatam says
you can try the second circuit published in this article:
https://www.homemade-circuits.com/2012/07/make-6v-4ah-automatic-battery-charger.html
Swagatam says
12v input will not charge a 12V battery